This invention generally pertains to formulations and methods for producing expanded polymeric bodies (foams). More particularly, the present invention recites formulations and methods for producing low density expanded polymeric bodies which are uniformly dense from edge-to-edge, and specifically where the nominal density varies by less than about xc2x16% across any cross-section of the body.
Many polymer formulations are known in the art for providing polymeric foam structures. In particular, porous polymeric foams have been produced by a wide variety of techniques ranging from leaching a soluble filler phase through providing an internally generated gas-producing phase, to utilizing emulsion-derived phase separation techniques. Some of the most commercially important polymer systems are drawn to polyurethane foams. A large body of information relating to the preparation of rigid and flexible polyurethane foams has been published over the years. For example, Windemuth in U.S. Pat. No. 2,948,691 issued on Aug. 9, 1960 teaches that flexible polyurethane foams can be made from mixtures comprising aliphatic or aromatic isocyanates and polyols. Britain in U.S. Pat. No. 3,397,158 issued Aug. 13, 1968 and Hostettler in U.S. Pat. No. 3,398,106 issued Aug. 20, 1968 both describe an improvement in polyurethane foam making processes through the use of stannous octoate catalyst. Other important foam structures are derived from emulsion techniques, such as are described in U.S. Pat. Nos. 4,775,655 and 5,021,462 to Edwards, et al., and to Elmes, et al., respectively, in order to produce materials such as polystyrene, and polymethacrylonitrile. Finally, International Application WO 00/37547, published under the Patent Cooperation Treaty (PCT), discloses a method for producing thermally expandable polymeric microspheres and a material formed by expanding a quantity of the microspheres at an elevated temperature such that the walls of the expanded microspheres fused together.
However, none of the prior art discloses a method for providing an expanded foam body having a density which can be controlled edge-to-edge to within a small tolerance range. When these materials are prepared to produce a foam body, control of various physical parameters, such as the density of the end product, is problematicxe2x80x94typically limited to achieving a nominal average value, determined by trial and error. The end result is usually found to vary widely across a cross-section of the body. Furthermore, many of the materials used by the prior art methods, are messy, noxious, and/or toxic and require either an intense hand operation or sophisticated and expensive mixing/dispensing equipment in order to prepare the ingredients for generating the foam structures.
It would be desirable then to provide a method for providing a simple, clean, and inexpensive means for producing a foam structure having well controlled physical properties.
In the present invention, therefore, there is provided a method for preparing and producing a constrained foam structure having well-controlled dimensional tolerances, and density, especially with regards to density gradients near a mold wall. A simple mixture of commercially available dry materials is provided. It is, therefore, an object of the present invention to provide a method for preparing an organic foam structure having an internal density variation of less than about xc2x16% of the nominal bulk density of the gross structure.
Yet another object of the present invention is to provide a composition for providing an organic foam structure having an internal total density gradient of less than about 12% of the nominal bulk density everywhere within the foam structure.
Another object of the present invention is to provide a composition comprising a first fraction of expanded and a second fraction of unexpanded thermally expandable polymeric microspheres.